OBJECTIVES: 1. A number of aspects of intracellular pH regulation will be studied in barnacle muscle fibers. Measurement of pHi will be with Hinke-type intracellular pH-sensitive microelectrodes. The study will particularly be concerned with facilitated proton transport accomplished by weak acids, and with a quantitative investigation of the parameters of the rate of "proton pumping" (or its equivalent). 2. Thomas-type pH-sensitive microclectrodes will be constructed. These electrodes will be used to investigate intracellular pH and its determinants in frog and rat muscle fibers.